1. Field of the Invention
The present invention relates to a sensor using a field effect transistor (FET).
2. Description of the Related Art
Japanese Patent No. 3,313,696 (Patent Document 1) discloses a chemical sensor using an FET, and U.S. Pat. No. 4,180,771 (Patent Document 2) discloses a chemical sensor in which an FET and a reference electrode are combined. However, the related arts described above have some problems, which will be described below in detail.
FIG. 1 is a schematic diagram showing a sensor using an FET having one gate electrode, which is described in Patent Document 1. The device has a source electrode 5, a drain electrode 6, an insulating film 7, a target substance 9, and a non-target substance 10. When a voltage is applied to a gate electrode 2 shown in FIG. 1, a channel 13 is formed in an upper layer region of a semiconductor layer 4. In such a case, since the channel 13 and a receiving layer 8 are in close proximity to each other, it is expected that measurement can be performed with high sensitivity. However, because of the close proximity between the channel 13 and the receiving layer 8, carrier movement occurs from the channel 13 to a sample solution 11. That is, leakage current occurs from the channel 13 to the sample solution 11. Consequently, it is difficult to perform electrical measurement accurately.
FIG. 2 is a schematic diagram showing another sensor including one gate electrode and a reference electrode, which is described in Patent Document 2. In the sensor shown in FIG. 2, since the electrical potential of a sample solution 11 can be kept constant by a reference electrode 12, it is believed to be possible to perform stable electrical measurement. However, since a channel 13 is formed by applying a voltage to a gate electrode 200 having a gate-insulating layer 3 thereon, the channel 13 is formed in a lower layer region of a semiconductor layer 4 at a point distant from a receiving layer 8. Consequently, it is difficult to perform electrical measurement with high sensitivity.